In conventional acoustic well logging operations, the velocity of an acoustic wave propagating through an earth formation traversed by a well may be determined. Throughout this specification, the synonymous terms "well" and "borehole" will be used interchangeably, and the phrases "acoustic wave" and "acoustic signal" will be used in their general sense to denote any compressional wave, shear wave, guided acoustic wave, or any other elastic wave. A conventional acoustic well logging system includes: a logging sonde that may be suspended downhole in the borehole fluid, a source contained within the sonde for generating compressional waves in the borehole fluid, and one or more detectors within the sonde and spaced apart from the compressional wave source for detecting compressional waves in the borehole fluid. Compressional-wave energy in the borehole fluid generated by the source is refracted into the earth formation surrounding the borehole.
Some of the energy in the compressional waves in the fluid is refracted into the formation surrounding the borehole. Some of the refracted energy then propagates in the formation as a refracted compressional wave, and some propagates in the formation as a refracted shear wave. Another portion of the energy radiated by the compressional-wave source is converted into the form of guided waves that travel in the borehole fluid and the part of the formation adjacent to the borehole. A portion of the energy in each refracted compressional wave and shear wave is refracted back into the borehole fluid in the form of compressional waves and reaches the detector in the logging sonde. The guided waves are also detected by such detector. Any wave that is one of these three types of waves detected by the detector may be called an arrival. The compressional-waves in the borehole fluid caused by refraction of compressional waves in the formation are called the compressional wave arrivals; those caused by refraction of shear waves in the formation are called the shear-wave arrivals; and those caused by guided waves are called the guided-wave arrivals. Thus, if the signal generated by the source is an impulsive signal, the signal detected by the detector is a composite signal which includes a number of impulsive components including the compressional-wave arrival, the shear-wave arrival and the guided-wave arrivals. In earth formations compressional waves travel faster than shear waves, and shear waves in the formation usually travel faster than the guided waves. Therefore, in the composite signal detected by the detector, the compressional-wave arrival is the first arrival, the shear-wave arrival is the second arrival, and the guided-wave arrivals are the last arrivals. In measuring the compressional-wave velocity of the formation, the time interval between generation of compressional waves and detection of the first arrival by the detector gives the approximate travel time of the refracted compressional wave in the formation. Hence the later shear-wave and guided-wave arrivals do not affect measurement of the compressional-wave velocity of the formation. The ratio of the distance between the source and detector to the time between generation and detection of the energy in the compressional-wave arrival yields the velocity of compressional waves in the formation. The distance between source and detector is usually fixed and known so that measurement of the time between compressional-wave generation and detection of the compressional-wave arrival is sufficient to determine the velocity of compressional waves in the formation. For better accuracy, such distance is usually much greater than the dimensions of the source or detector. Alternatively, measurement of the time interval between the detections of a compressional-wave arrival, at two detectors separated by a known distance, can be used to measure the velocity of compressional-waves in the formation.
Information important for production of oil and gas from subterranean earth formations may be derived from the compressional-wave velocities of such formations. It is also known that determination of the velocity of shear waves may yield information important for production of oil and gas from the formations. The ratio between the shear-wave velocity and compressional-wave velocity may reveal the lithology of the subterranean earth formations. The shear-wave velocity log may also enable seismic shear-wave time sections to be converted into depth sections. The shear-wave log is also useful in determining other important characteristics of earth formations such as porosity, fluid saturation and the presence of fractures.
Conventional compressional-wave logging sources of the monopole type generate compressional waves that are symmetrical about the axis of the logging sonde. When such monopole compressional waves are refracted into the surrounding earth formation and detected with conventional receivers of the monopole type, the relative amplitudes of the refracted monopole shear and compressional waves are such that it is difficult to distinguish the later shear-wave arrival from the earlier compressional-wave arrival and its reverberations in the borehole.
However, it has been proposed that a multipole transmitter-detector pair (i.e., a dipole-source/dipole-receiver pair, a quadrupole-source/quadrupole-receiver pair, or a higher-order-multipole-source/receiver pair where the multipole order of the source matches that of the receiver) be used in a well logging system to facilitate direct shear-wave velocity logging. Such a multipole well logging system, operated at the proper frequency, will produce arrivals at the detector such that the amplitude of the detected shear-wave arrival is significantly higher than that of the compressional-wave arrival. By adjusting the triggering level of the detector (and the system for recording the detected signal) to discriminate against the compressional-wave arrival, the shear-wave arrival is detected as the first arrival. Dipole acoustic wave well logging systems of this type are disclosed in U.S Pat. No. 4,606,014 issued Aug. 12, 1986 to Winbow, et al.; European Patent Application No. 031,989 by Angona, et al. (published July 15, 1981); and U.S. Pat. No. 3,593,255 issued July 13, 1971 to White. However, the prior art (including the cited references) teaches that the source and receiver (or receivers) of a dipole (or higher order multipole) system should be aligned so that each source and receiver is associated with substantially the same azimuthal angle relative to the borehole's longitudinal axis (i.e., the azimuthal angle between the source and receiver is 0.degree.) so as to maximize the sensitivity of the system. Similarly, the references teach that if the azimuthal angle between a dipole source and a dipole receiver is 90.degree., the receiver will be insensitive to dipole wave energy produced by the source, and that if the angle between a quadrupole source and a quadrupole receiver is 45.degree., the receiver will not detect quadrupole radiation produced by the source.
Multipole transducers of the quadrupole, octopole, and higher-order multipole types are described in the following Applications and Patent, all assigned to the same assignee as is the present Application: U.S. patent application Ser. No. 379,684, filed May 19, 1982; U.S. patent application Ser. No. 440,140, filed Nov. 8, 1982; and U.S. Pat. No. 4,649,526, issued Mar. 10, 1987 to Winbow, et al.
It has for some time been known that thinly bedded horizontal formations and horizontally fractured rocks exhibit transverse isotropy. In this situation, the velocity of compressional and shear waves generally depends on their direction of propagation with respect to the vertical. However, their velocity is independent of the azimuthal direction in which they propagate. Alternatively, the formation may exhibit azimuthal anisotropy, in which compressional waves may travel at different speeds in different azimuthal directions away from (or toward) a vertical borehole. Similarly, the speeds of shear waves depend on the azimuthal direction (relative to the borehole axis) in which they propagate. Azimuthal anisotropy may be caused by vertical fractures, among other geologic factors.
In an azimuthally anisotropic medium, the velocity of a shear wave also depends on the polarization direction (i.e., the plane containing the particle motion). For example, the velocity of a vertically propagating shear wave whose polarization is North-South will in general differ from the velocity of a vertically propagating shear wave whose polarization is East-West.
In their simplest form, azimuthally anisotropic media have five independent elastic constants associated with them as compared with two independent elastic constants for fully isotropic media. In principle, more complex types of anisotropy are also possible, and describing those forms of anisotropy may require as many as 21 independent elastic constants.
Azimuthal anisotropy has been reported to be a widespread phenomenon. Even a small amount of azimuthal anisotropy (for example, 3%), where the amount of azimuthal anisotropy is defined to be .epsilon.=.vertline.(V.sub..perp. -V.sub..parallel.)/V.sub..parallel. .vertline..multidot.100% where V.sub..parallel. =velocity of wave propagating with polarization parallel to a selected direction, and V.sub..perp. =velocity of wave having the same frequency content but having polarization perpendicular to the selected direction (for example, parallel and perpendicular to fracture orientation V.sub..parallel. and V.sub..perp. will be slow and fast velocities) has a significant effect on correlating shear-wave seismic data.
If azimuthal anisotropy could be detected and quantified during a well logging operation, the information could be used to help interpret direct hydrocarbon indicators, locate and define fractured reservoirs, and deduce lithologic information from seismic data. Azimuthal anisotropy data, gathered during such a well logging operation, might also be used in performing production studies of fractured reservoirs since the information regarding azimuthal anisotropy is indicative of the existence and orientation of vertical fractures provide high-permeability pathways for hydrocarbons and reservoir quality.
Until the present invention, it has not been recognized how the effects of azimuthal anisotropy may be measured using well-logging tools.